When producing biaxially oriented tubing of thermoplastic material, e.g. pipes of polyvinylchloride, it has proven to be difficult to produce tubing with uniform final dimensions. Such uniformity is desirable, e.g. as biaxially oriented tubing elements, e.g. pressure pipes, e.g. for transportation of water, are interconnected end-to-end, e.g. via socket connections.
The first aspect of the invention relates to a method for producing a biaxially oriented tube from thermoplastic material, wherein a tube in preform condition is extruded from thermoplastic material using an extruder which is provided with an extruder die head having an inner die member, the inner die member forming a lumen in the tube in preform condition, wherein the tube in preform condition is subjected to a temperature conditioning, so that a tempered tube in preform condition is obtained having an orientation temperature which is suitable for the thermoplastic material, and wherein use is made of an expansion device, said expansion device comprising:                a non-deformable expansion part having a gradually increasing diameter to a maximum diameter at a downstream end thereof, which expansion part is contacted by the tube and exerts an expanding force on the tube so as to bring about an expansion of the tempered tube in preform condition in circumferential direction,        a run-on part which is located upstream of the expansion part, said run-on part having an upstream nose end.        
The method comprises drawing the tempered tube over the expansion device using a drawing device which is arranged downstream of the expansion device and acts on the tube, in such a manner that said tube is transformed from a tube in preform condition into a biaxially oriented tube with thermoplastic material which is oriented in axial direction and in circumferential direction of the tube. The biaxially oriented tube is cooled.
In this method of the first aspect of the invention use is made of an expansion device having one or more fluid supply ducts. The one or more fluid supply ducts have a port in the outer surface of the run-on part and/or the expansion part of the expansion device, and a fluid is introduced between the expansion device and the tube.
First some prior art approaches will be discussed here.
In EP 823 873 a method is disclosed for the production of biaxially oriented tubing. Use is made of a rigid mandrel having an expansion part as well as a run-on part upstream of and integral with the expansion part. Spaced upstream from the nose end of the run-on part a closure member is held on the anchoring rod, so as to define a chamber in the lumen of the tube in preform condition. A liquid, e.g. heated water, is fed under pressure between the tube and the mandrel device via one or more ducts that are formed in the mandrel and have a port in the outer surface of the mandrel. This liquid then flows counter to the direction of movement of the tube towards the chamber upstream of the nose end of the expansion device and is then discharged via one or more discharge ducts in the anchoring rod.
In EP 823 873 it is also proposed to provide the mandrel with a run-off part downstream of the expansion part. A film of cold liquid is created between the tube and this run-off part, as one or more feed and discharge ducts for said cold liquid are formed in the mandrel. In particular it is proposed to cause the cold liquid in said film to flow opposite to the motion of the tube, so from a downstream feed opening in the outer surface of the run-off part towards an upstream discharge opening in the outer surface of the run-off part.
In EP 823 873 the tube is made to sealingly engage the mandrel at or near the transition between the expansion part and the run-off part in order to avoid that the cold liquid reaches the expansion part.
In EP 1 159 122 a method is disclosed for the production of biaxially oriented tubing. Use is made of a rigid mandrel having an expansion part as well as a run-on part upstream of and integral with the expansion part. The run-on part has a uniform diameter over its length. A film of liquid is formed between the expansion part and the tube. The liquid is supplied at the downstream end of the expansion part and flows counter to the motion of the tube to one or more outlets arranged in the run-on part of the expansion device. The tube in preform condition is shown to sealingly engage on the nose end of the run-on part as the inner diameter of the preform is less than the diameter of the run-on part.
The first aspect of the invention aims to provide measures that allow for improvements over the prior art or at least provide for a useful alternative.
It is a further object of the first aspect of the invention to provide for measures that allow for a suitable internal tempering of the tube in preform condition, possibly using liquid circulated within the lumen, e.g. in combination with heating and/or cooling on the outside of the tube in preform condition.
It is a further object of the first aspect of the invention to provide measures that allow for a suitable internal tempering of the tube in preform condition, possibly using liquid circulated within the lumen, as well as introducing a fluid, e.g. a liquid or a gas, between the expansion device, e.g. the expansion part thereof, and the tube, the introduction of fluid and the internal tempering being independent from one another.